The present invention relates to a method for producing polybutylene terephthalate of high quality through continuous direct esterification followed by polycondensation with accompanied by little side reaction therethrough, and to polybutylene terephthalate with good stiffness.
Having good moldability, heat resistance, good mechanical properties and chemical resistance, polybutylene terephthalate (hereinafter referred to as PBT) is widely used as a material to be formed into electric parts and car parts. In addition, being soft and stretchable, PBT is also widely used as a material of fibers.
One method of producing such PBT, that is known is a direct polymerization method comprising an esterification step of esterifying terephthalic acid with 1,4-butanediol to give a PBT oligomer followed by a polycondensation step of further polymerizing the resulting PBT oligomer to a high degree of polymerization. In this direct polymerization method, the esterification step indispensably requires a catalyst. As the catalyst, organic titanium compounds are most popularly used. For example, Japanese Patent Application (JP-A) No. Sho-52-51495 discloses a continuous direct polymerization method for producing PBT, which uses a titanium catalyst.
However, though displaying excellent catalytic activity in esterification, organic titanium compounds (e.g., titanates) are problematic in that they are often hydrolyzed with ease by water that is produced during esterification and through decomposition (dehydrating cyclization) of 1,4-butanediol, thereby forming insoluble impurities and lowering their catalytic activity.
In order to solve this problem of insolubilization and inactivation of organic titanium compounds, various methods have heretofore been proposed, including, for example, a method of adding an organic titanium compound in two stages, or that is, adding it in both the former stage, in which the degree of esterification has reached 50%, and the latter stage (for example, JP-A Sho-49-57092); a method of using an organic titanium compound along with a polyhydric alcohol compound and an alkali metal compound (for example, JP-A Sho-62-225524); a method of using an alkanolamine titanate compound as the catalyst (for example, JP-A Sho-62-141022); a method of using an organic titanium compound along with an alkanolamine compound (for example, JP-A Sho-62-199617); and a method of adding a hindered phenol compound in the stage of esterification (for example, JP-A Hei-1-282215). In those methods, the catalytic activity of the catalysts being used could be improved in some degree.
However, though being effective in batch wise production of PBT oligomers, those methods are not so much effective in continuous production of PBT oligomers as producing insoluble impurities in large quantities. This may be because, in continuous production of PBT oligomers, organic titanium compounds will react not only with water but also with terephthalic acid in the reaction system to be inactivated while forming insoluble metal salts.
This problem of the formation of insoluble impurities and the reduction in the catalytic activity in the esterification stage shall naturally be a factor of interfering with the reduction in the reaction time, while inducing other various problematic matters such as the transparency of the reaction product is much lowered, that the intended degree of polymerization could not be obtained due to the reduction in the polymerization reactivity, that the moldability of the polymer product is lowered due to the insoluble impurities inevitably existing in the polymer, and even that the quality, especially the toughness of the shaped articles of the polymer is lowered.
Except for organic titanium compounds, Japanese Patent Publication (JP-B) No. Sho-53-9796 discloses organic tin compounds which are said to be effective. However, the disclosed organic tin compounds are problematic in that the continuous esterification using them is dull while the amount of the side product formed is increased, and that, if an increased amount of the catalyst is added in order to promote the reaction, the polymer product, PBT is discolored and its heat resistance is lowered.
JP-A Sho-62-195017 discloses continuous direct polymerization of 1,4-butanediol and terephthalic acid in a molar ratio, the former to the latter, of not smaller than 2, in which is used a titanium or tin catalyst.
However, continuous esterification in a high molar ratio is problematic in that side reaction and introduction of ether bonds into the molecular chain being formed easily to occur. Therefore, it is desirable that the molar ratio is as small as possible, but in the disclosed continuous direct esterification, it is difficult to reduce the molar ratio.